Acts as an actin nucleation factor, remains associated with the slow-growing pointed end of the new filament (PubMed:11747823, PubMed:21620703). Involved in intracellular vesicle transport along actin fibers, providing a novel link between actin cytoskeleton dynamics and intracellular transport (PubMed:11747823). Required for asymmetric spindle positioning and asymmetric cell division during meiosis (PubMed:21620703). Required for normal formation of the cleavage furrow and for polar body extrusion during female germ cell meiosis (PubMed:21620703). Also acts in the nucleus: together with FMN2, promotes assembly of nuclear actin filaments in response to DNA damage in order to facilitate movement of chromatin and repair factors after DNA damage (PubMed:26287480). In addition, promotes innate immune signaling downstream of dsRNA sensing (PubMed:35148361). Mechanistically, contributes to IRF3 phosphorylation and activation downstream of MAVS and upstream of TBK1 (PubMed:35148361). {ECO:0000269|PubMed:11747823, ECO:0000269|PubMed:21620703, ECO:0000269|PubMed:26287480, ECO:0000269|PubMed:35148361}. This is the function of SPIRE1 (spire type actin nucleation factor 1, ENSG00000134278).